1. Field of the Invention
The present invention relates to a para-aramid fiber, a para-aramid pulp and processes for producing the same. In particular, the invention relates to a dope constituted of a para-aramid of low degree of polymerization, a para-aramid fiber and a para-aramid pulp produced from said dope, and processes for producing these materials.
2. Related Art
Para-oriented aromatic amides which are condensation polymers of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide (hereinafter abbreviated to "para-aramids") have hitherto been known to be useful in various fields such as fiber, pulp and the like because of their high strength, high elastic modulus and high heat resistance.
As a typical member of para-aramid, poly(para-phenylene terephthalamide) (hereinafter abbreviated to PPTA) can be referred to.
Hitherto, PPTA pulp has been produced in the following manner. Thus, PPTA is produced by carrying out a solution polymerization reaction in a polar amide solvent such as N-methyl-2-pyrrolidone (hereinafter abbreviated to NMP) or the like. The PPTA is re-precipitated, neutralized, washed with water and dried, and once isolated as a polymer. Then, the polymer is dissolved in a solvent and made into a PPTA fiber by the process of wet spinning. In this step, concentrated sulfuric acid is used as the solvent of spinning dope, because PPTA is not readily soluble in organic solvents. This spinning dope shows an optical anisotropy as mentioned in Japanese Patent Application KOKOKU No. 50-8474.
On the other hand, in some limited cases, a solution of para-aramid in an organic solvent also exhibits an optical anisotropy. Thus, Japanese Patent Application KOKOKU No. 50-35941 discloses an optically anisotropic spinning dope exemplified by a solution of 2,6-dichloro-paraphenylene 2,6-naphthalamide polymer in N,N-dimethylacetamide/lithium chloride system and the like. Japanese Patent Application KOKOKU No. 50-12485 discloses an optically anisotropic spinning dope prepared by dissolving PPTA which is a typical para-aramid in hexamethylphosphoramide/N-methylpyrrolidone/lithium chloride system. The former spinning dope is characterized by using a monomer having an affinity to organic solvents, while the latter spinning dope is characterized by using hexamethylphosphoramide having a high ability to dissolve para-aramid.
Industrially, PPTA fiber is produced from a spinning dope using concentrated sulfuric acid as a solvent, considering the performances as a long fiber, particularly strength and stiffness.
According to the prior process, a pulp is produced by mechanically cutting a PPTA fiber, dispersing the cut fiber in water and fibrillating the dispersed fiber by a mechanical shearing means such as beating or the like, followed by filtration and drying. In such prior process, the steps of polymerization, spinning and pulp making are completely independent of one another. That is, the step of polymerization uses a polar amide solvent, the .step of spinning uses concentrated sulfuric acid as solvent, and the step of pulp making uses water as a dispersing medium. This is economically disadvantageous as an industrial process.
With the aim of rationalizing the prior process, there have been proposed up to date various processes for directly spinning a yarn or making a pulp from a liquid polymer dope without separating the step of polymerization and the step of spinning from each other.
Thus, Japanese Patent Application KOKAI No. 2-242912 proposes a process which comprises subjecting a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide to a polycondensation reaction at an equimolar ratio in a polar amide solvent, gelling the polymer solution which is in a stage just before completion of the polymerization and exhibits an optical anisotropy while orienting and making flow the solution, thereafter cutting the gel into pieces, and making the pieces into a pulp by using water as a coagulant while applying thereto a shearing force. In this process, the polymerization can be made to progress to an additional extent by maintaining the cut gel pieces at a high temperature, and thereby a high molecular weight para-aramid pulp is formed as a final product.
Japanese Patent Application KOKOKU No. 3-29883 proposes a process for producing a fiber which comprises subjecting an aromatic diamine and an aromatic dicarboxylic acid chloride to a polycondensation reaction at an equimolar ratio in a polar amide solvent in which an inorganic salt is dissolved, and spinning the optically anisotropic polymer solution just before completion of the polymerization, as a spinning dope, in a coagulating bath containing a tertiary amine. The patent gazette makes mention of a process for producing a pulp from the fiber, too. According to the process of the patent, the polymerization further progresses after the spinning by the action of the tertiary amine present in the coagulating solution and finally gives an aramid fiber or pulp of high degree of polymerization.
Apart from the above-mentioned proposals, Japanese patent Application KOKOKU No. 57-10885, Japanese Patent Application KOKOKU No. 61-42004, Japanese Patent Application KOKAI No. 62-45716 and Japanese Patent Application KOKAI (Laid-Open) No. 62-162013 also propose production processes which are fundamentally similar to the processes of the above-mentioned patents. That is, in all these processes, a fiber or a pulp is produced by spinning a polymer solution exhibiting an optical anisotropy which is in a stage just before completion of the polycondensation, or by adding a coagulant solution to the polymer solution, and further applying a high shearing force thereto. In any of these cases, an aromatic diamine and an aromatic dicarboxylic acid halide are subjected to a polycondensation reaction at an equimolar ratio, and a further polymerization is made to progress either in parallel with the spinning or pulp-making or just after the spinning or pulp-making to obtain a high-molecular weight aramid fiber or pulp as a final product.